In a prior-art AC-to-DC converter circuits employing thyristors or diodes, the output of the converter circuit or the rectified voltage as well as the rectified current pulsates under the influence of the AC voltage waveform of a power source, and hence, the pulsation is diminished by the use of a filter or a smoothing reactor. In this case, since the change of an AC current on the power source side of a transformer during the commutation period of the thyristor or diode is abrupt, high frequency components included in the AC current are large. Where the converter circuit is used for the control of a motor vehicle which is driven by the single-phase alternating current, these high frequency components can cause various problems including inductive disturbances to nearby communications equipment. In order to eliminate these problems, much effort has been expended up to now.
One method considered has been to moderate the change of the AC current on the power source side during the commutation period and to thus reduce the high harmonic frequency components included in the current. To this end, an inductance is incorporated into the AC circuit. However, when the inductance is made large, the power factor usually lowers to result in lowering the rectified voltage.
Where the supply voltage is, for example, sinusoidal, the change of the AC current on the power source side as attendant upon the commutation is abrupt when the commutation is executed near 90.degree. in terms of the phase of the supply voltage, and it is moderate when the commutation is executed near 0.degree. or 180.degree..